To enhance and maintain wildlife benefits of Conservation Reserve Program( CRP) lands, managers and policy makers need information on how grassland succession affects vegetation structure and composition as fields age. We describe changes in vegetation on 154 northern Missouri CRP fields sampled in 1989-1995 (field ages 1 to 9 years), including differences between cool-season grass and warm-season grass plantings. Within 3-4 years, CRP fields became dominated by perennial grasses with substantial litter accumulation. These vegetation conditions may limit the value of enrolled lands for many wildlife species. Once established, warm-season grass fields were taller and had more bare ground than cool-season fields. However, height and density of vegetation on the warm-season grass fields may have precluded use by many wildlife species, even grassland birds commonly associated with tall-grass habitats. Mowing, the primary disturbance during our study, had very short-term effects on vegetation structure and led to accelerated grass succession and litter accumulation. Thus, mowing did little to set back succession, increase diversity, and maintain or increase wildlife benefits. With wildlife benefits having an increased role in the current CRP, program administrators and cooperating agencies must recognize that management is necessary to maintain, and can even enhance, the wildlife benefits ascribed to specific CRP plantings.